Stable carbon and oxygen isotopic signatures in soil carbonate nodules for paleoclimate reconstruction in southwestern Taiwan

Abstract Stable carbon and oxygen isotopes in carbonate nodules were controlled by the type of vegetation (C 3 or C 4 ) and soil temperature, respectively. These signatures can reflect the paleoclimatic conditions during the formation of the carbonate nodules. Carbonate nodules were sampled along a soil chronosequence in southwestern Taiwan, representing different ages of shoreline changes associated with phases of marine transgression and regression. The stable carbon and oxygen isotopes in carbonate nodules were analyzed. The ages of the carbonate nodules were determined by radiocarbon ( 14 C) dating. The climofunctions of mean annual precipitation and temperature were calculated to interpret the climatic conditions. The carbonate nodules were relictic pedofeatures that formed episodically under favorable pedogenic conditions between 4063 and 690 years before present (years BP). The δ 13 C values indicated a mixed vegetation of C 3 and C 4 plants, with a predominance of C 4 plants (68.0 to 98%). The mean annual temperature estimated by the climofunction of δ 18 O value (12.3 to 14 °C) was lower than the present (24.7 °C). The mean annual precipitation (1036 to 1342 mm yr –1 ) calculated by the geochemical climofunction was lower than the present (1829 mm yr –1 ). The climate was drier and cooler during this period, and this may be due to a weaker East Asian Summer Monsoon, which favored calcification in the soils. The climatic conditions, as interpreted by the stable carbon and oxygen isotopes of carbonate nodules, were consistent with the calcification process.